The Union Pacific Challengers were simple articulated 4-6-6-4 steam locomotives built by American Locomotive Company for the Union Pacific Railroad. Built between 1936 and 1943, the Challengers were nearly 122 feet long and weighed more than one million pounds. They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to passenger trains operating through mountain territory to California and Oregon. The locomotives were built specifically for Union Pacific and much of the experience gained later went into the design of the "Big Boy".

The name "Challenger" was given to steam locomotives with a 4-6-6-4 wheel arrangement. This means that they have four wheels in the leading pilot truck, which helps guide the locomotive into curves; two sets of six driving wheels, and finally, four trailing wheels, which support the rear of the engine and its massive firebox. Each set of driving wheels has its own steam cylinder. In essence, the result is two engines under one boiler.